The present invention relates generally to restricted flow passages and to piston and slipper arrangements used in hydraulic pumps and motors of either the axial or radial piston type and of either fixed or variable displacement, and more particularly relates to a piston structure which utilizes a novel restricted flow passage to control the flow of fluid to the slipper thrust or bearing surface.
In order to reduce power loss and wear due to the contact between a piston slipper and a reaction member, it has been known to provide a fluid passageway through the piston to a recess in the thrust face of the piston slipper so that fluid pressure at the inner end of the piston is also available at the recess and acts between the slipper and reaction member to exert a separating force between the two and provide a film of oil on which the slipper rides. The ideal arrangement is to size the recess in the slipper so as to obtain a complete hydrostatic balance without any significant leakage. That is, the force exerted by the fluid pressure at the inner end of the piston should be exactly equal to the force exerted by the fluid pressure between the slipper and reaction member and there should be no appreciable leakage across the land of the slipper. However, because of manufacturing tolerances and varying operating conditions, such an idealistic arrangement is difficult to achieve by proper sizing of the slipper recess.
It has been proposed to overcome the above described problem by providing a sufficiently large recessed area in the slipper so the force at the slipper is, at all times, at least as great as the force at the inner end of the piston, and to provide a restriction in the flow passage through the piston so that whenever excessive leakage occurs there will be a pressure drop across the restriction with the result that the fluid pressure separating the slipper from the reaction member will be reduced and the slipper will settle down to its normal position. Examples of this are shown in U.S. Pat. No. 3,828,654 which issued to Roger H. Wiethoff on Aug. 13, 1974 and U.S. Pat. No. 3,188,973 which issued to Firth et al. on June 15, 1965. The major problems with such a solution is that in order to obtain the necessary pressure drop through the piston the restriction is so small it is subject to plugging, and plugging would result in a loss of pressure at the recess and a rapid failure of the slipper and reaction member.